我正在尝试在 Xamarin.Forms(针对 Android 平台)上的 ImageView 上设置阴影，并且我在互联网上获得了一些示例。

PCL代码很简单，平台看起来也很简单。 The recipe available at the official xamarin developer site是这样的:

[assembly: ResolutionGroupName("MyGroupName")]
[assembly: ExportEffect(typeof(LabelShadowEffect), "ShadowEffect")]
namespace MyWorkspace
{
    public class LabelShadowEffect : PlatformEffect
    {
        protected override void OnAttached()
        {
            try
            {
                var control = (Control as TextView); // TextView have the SetShadowLayer method, but others views don't

                var effect = (ShadowEffect)Element.Effects.FirstOrDefault(e => e is ShadowEffect);
                if (effect != null)
                {
                    float radius = effect.Radius;
                    float distanceX = effect.DistanceX;
                    float distanceY = effect.DistanceY;
                    Android.Graphics.Color color = effect.Color.ToAndroid();
                    control?.SetShadowLayer(radius, distanceX, distanceY, color);
                }
            }
            catch (Exception)
            {               
            }
        }

        protected override void OnDetached() 
        { 
        }
    }
}

所以我注意到这个秘诀仅适用于使用 TextView 呈现的组件(这是唯一具有 SetShadowLayer 方法的类)。在 other sources我看到了一些更通用的东西，比如:

public class ShadowEffect : PlatformEffect
{
    protected override void OnAttached ()
    {
        Container.Layer.ShadowOpacity = 1;
        Container.Layer.ShadowColor = UIColor.Black.ToCGColor;
        Container.Layer.ShadowRadius = 6;
    }

    protected override void OnDetached ()
    {
        Container.Layer.ShadowOpacity = 0;
    }
}

通过使用 UIColor 我知道它是针对 iOS 平台的。在 Android View 上没有这样的事情。我查看了 XF FrameRenderer 源代码，但无法理解它们是如何使阴影效果起作用的。

有人可以帮我吗？

最佳答案

遗憾的是，没有直接的方法让它在 Android 上运行。但是您可以尝试一些选项。

选项 #1

There are several unsupported drawing operations for hardware accelerated layers ，其中包括用于非 TextView 的 SetShadowLayer。

因此，为了获得非 TextView 的 SetShadowLayer 渲染，您需要将 LayerType 渲染设置为 SOFTWARE 作为在此solution中解释.

SetLayerType(LayerType.Software, null);

但主要缺点当然是它可能是性能问题。

选项 #2

第二种选择是使用径向渐变来模拟阴影。我已将其实现为渲染器(但您也应该能够将其实现为效果)。结果当然不如模糊阴影效果那么好。您还必须设置正确的 Padding 以留出一些空间让阴影呈现，并在图像下方可见。

protected override void DispatchDraw(global::Android.Graphics.Canvas canvas)
{
    try
    {
        var nativeCtrl = Control;
        var formsElement = Element;
        if (nativeCtrl == null || formsElement == null)
        {
            base.DispatchDraw(canvas);
            return;
        }

        //convert from logical to native metrics if need be
        var shadowDistanceX = 10f;
        var shadowDistanceY = 10f;
        var shadowRadius = 5f;
        var shadowOpacity = .5f;
        var shadowColor = Color.Black;
        var cornerRadius = 0.2f;

        var bounds = formsElement.Bounds;

        var left = shadowDistanceX;
        var top = shadowDistanceY;
        var right = Width + shadowDistanceX;
        var bottom = Height + shadowDistanceY;

        var rect = new Android.Graphics.RectF(left, top, right, bottom);

        canvas.Save();
        using (var paint = new Android.Graphics.Paint { AntiAlias = true })
        {
            paint.SetStyle(Android.Graphics.Paint.Style.Fill);

            var nativeShadowColor = shadowColor.MultiplyAlpha(shadowOpacity * 0.75f).ToAndroid();
            paint.Color = nativeShadowColor;

            var gradient = new Android.Graphics.RadialGradient(
                0.5f, 0.5f,
                shadowRadius,
                shadowColor.ToAndroid(),
                nativeShadowColor,
                Android.Graphics.Shader.TileMode.Clamp
            );
            paint.SetShader(gradient);

            //convert from logical to native metrics if need be
            var nativeRadius = cornerRadius;  
            canvas.DrawRoundRect(rect, nativeRadius, nativeRadius, paint);

            var clipPath = new Android.Graphics.Path();
            clipPath.AddRoundRect(new Android.Graphics.RectF(0f, 0f, Width, Height), nativeRadius, nativeRadius, Android.Graphics.Path.Direction.Cw);
            canvas.ClipPath(clipPath);
        }
        canvas.Restore();
    }
    catch (Exception ex)
    {
        //log exception
    }

    base.DispatchDraw(canvas);
}   

选项 #3

另一种选择是使用 SkiaSharp for Forms - 即创建一个容器(或分层) View ，在 subview (图像)周围呈现阴影。您也可以让 SkiaSharp 渲染图像，或者在布局中嵌入基于 XF 的图像控件。

protected override void OnPaintSurface(SKPaintSurfaceEventArgs args)
{
    var imgInfo = args.Info;
    var surface = args.Surface;
    var canvas = surface.Canvas;

    var drawBounds = imgInfo.Rect;
    var path = new SKPath();
    var cornerRadius = 5f;

    if (cornerRadius > 0)
    {
        path.AddRoundedRect(drawBounds, cornerRadius, cornerRadius);
    }
    else
    {
        path.AddRect(drawBounds);
    }

    using (var paint = new SKPaint()
    {
        ImageFilter = SKImageFilter.CreateDropShadow(
                                        offsetX,
                                        offsetY,
                                        blurX,
                                        blurY,
                                        color,
                                        SKDropShadowImageFilterShadowMode.DrawShadowOnly),
    })
    {
        canvas.DrawPath(path, paint);
    }
}

关于c# - 如何在ImageView上设置阴影效果，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/48460765/